1. Field of the Invention
The present invention relates to temperaturemonitoring apparatus and more particularly to such apparatus for monitoring the internal temperature of food being cooked in a microwave oven.
2. Description of the Prior Art
It is a desirable feature to include, in an oven, apparatus for monitoring the internal temperature of the food while it is being cooked. Such apparatus permits food to be cooked more accurately to a desired degree of doneness than would be the case either if a predetermined length of cooking time were employed or if a conventional mercury-type thermometer were employed. The former requires an accurate estimate of cooking time based upon oven temperature, in the case of a conventional oven, or upon microwave power level, in the case of a microwave oven, and, additionally, upon size and type of the food, in either a conventional or a microwave oven. The latter requires that the user of the oven periodically check the temperature reading, and is not readily adaptable for use in a microwave oven.
Electrical thermometers for monitoring internal temperature of food while it is being cooked have been developed for use in conventional ovens. Examples are disclosed in U.S. Pat. No. 3,778,798-Heit and U.S. Pat. No. 3,815,113-Welch. Electrical thermometers of the above-mentioned type comprise a needle-like probe adapted to be driven into the food, a temperature-sensing thermistor contained within the probe and located near the tip thereof, and a flexible cable connecting the probe to electrical circuitry within the oven, the electrical circuitry serving to either indicate the actual internal temperature of the food or to actuate a buzzer when a preset temperature is reached. In order that the electrical thermometer may be removed from the oven when it is not being used, the probe and cable are electrically connected to the electrical circuitry through a connector mounted on an interior wall of the oven cooking cavity.
Electric thermometer probes of the above-described type operate well in a conventional gas or electric oven. However, such probes, designed for use in a conventional oven, do not operate well when used in a microwave oven. The desired heating of food in a microwave oven occurs when the food, which is placed in the cooking cavity of the oven, "loads" or absorbs microwave energy, converting the energy into heat. Undesirably, food temperature-sensing probes of the conventional type also load in a microwave oven. This results in undesired high electrical currents flowing along the probe and cable and consequent undesired heating of the probe and cable. In particular, "hot spots" are produced at particular points along the cable and probe. This heating of the probe by direct action of the microwave energy, rather than solely by heat conducted from the food which is being cooked, causes the thermistor to sense a temperature which is higher than the actual internal temperature of the food. This results in a premature indication of doneness, a serious deficiency. Additionally, direct heating of the cable and the connector mounted on the interior wall of the cooking cavity results in a higher cable and connector temperature than would otherwise be the case. Such higher temperature can shorten the life span of the cable and the connector and may make them comfortably hot for the user of the oven to handle.
Such problems when a conventional temperaturesensing probe is used in a microwave oven can be minimized, but not eliminated, by measures such as providing the best possible thermal contact between the food being cooked and the probe so that heat produced in the probe by direct action of the microwave energy is dissipated, as much as is possible, into the food, or operating the oven at a lower microwave power level when the probe is being used. However, such measures are a compromise at best because some self-heating of the cable and probe still results and, further, the use of a lower microwave power level lengthens the required cooking time.
One approach to monitoring the temperature of a substance being heated in a high-frequency field is disclosed in an article by Kaftanov, S. D., et al., "Measuring the Temperature of Granulated Carbon Material in a High-Frequency Field," Ind. Lab. (USA), Vol. 38, No. 11, (November 1972), pp 1723-1724.
The article discusses some problems caused by direct heating of a temperature-sensing thermocouple by high-frequency energy. Additionally, the Kaftanov article discusses particular problems caused by arcing between adjacent carbon particles and between the probe tip and the carbon particles, which problems would not normally be present when the internal temperature of cooking food is being monitored. The Kaftanov article discloses a special thermocouple probe which includes a graphite screen filled with liquid tin to shield the thermocouple from direct heating by the high-frequency energy and to minimize arcing. Further, to shield the indicating circuitry from highfrequency interference, a special high-frequency filter comprising two shorted and two open-circuited sections of a coaxial quarter wavelength cable is inserted in series with each of the two thermocouple leads, and three LC sections in series are connected to the case of the indicating circuitry just before the thermocouple lead terminals. Such apparatus is complex and not readily adaptable for monitoring the internal temperature of food being cooked in a microwave oven.
U.S. Pat. No. 3,177,335-Fitzmayer et al. discloses a thermostat probe for measuring the ambient temperature in an oven of the type which cooks food using either conventional radiant energy or microwave energy, or both. The thermostat probe is used to control the operation of the conventional radiant heating energy elements. The thermostat is of the hydraulic type having a capillary tube leading into the oven cavity and terminating in a temperature-sensing bulb. It is necessary that the thermostat probe be insensitive to microwave energy and to respond only to ambient temperature within the oven cooking cavity. To this end, the probe is supported by a support having a plurality of spaced grounding points, the spacing between the grounding points being different than one quarter of the wavelength of the microwave energy or any odd multiple of one quarter wavelength of the microwave energy, so as to prevent the thermostat probe from creating a series-resonant loop at the frequency of the microwave energy. Such a configuration, relying upon a plurality of spaced grounding points, would not be suitable for a food temperature-sensing probe and cable assembly which, as a practical matter, is connected to the cooking cavity wall only at one end and to the food at the other end.
An example of food temperature-monitoring apparatus which is suitable for use in a microwave oven is disclosed and claimed in co-pending application Ser. No. 616,049, filed Sept. 23, 1975, by David Y. Chen and Louis H. Fitzmayer, and entitled "Food Thermometer for Microwave Oven," which application is a continuation-in-part of co-pending application Ser. No. 447,087, filed Mar. 1, 1974, each of the above mentioned applications being assigned to the same assignee as the present invention, and now abandoned. In the abovementioned Chen and Fitzmayer applications, there is disclosed apparatus including a temperature-sensing probe and a flexible sheilded cable connected at one end to the probe housing and at the other end to the wall of the cooking cavity. The total effective electrical length of the probe and the cable, measured along the cable and probe from the cooking cavity wall to the distal end of the probe, is selected to be an integer multiple of one-half the wavelength of the microwave energy. Resonant conditions, high attendant induced currents along the probe and cable, and direct heating of the probe and cable by the microwave energy are thereby minimized.
By the present invention, there are provided alternative forms of temperature-monitoring apparatus for monitoring the internal temperature of food while it is being cooked in a microwave oven, which alternative forms also minimize resonant conditions and attendant high induced currents and heating along the cable. In particular, the present invention is useful in a microwave oven operating at a frequency of 915 MHz.